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Interview of Alan Guth by Alan Lightman on 1987 September 21, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/34306-1
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Reasons why the inflationary universe has been so influential; history of the development of the inflationary universe model; high school interest in science and preference for oscillating universe model; early career in particle physics; change in career to cosmology; influence of Steven Weinberg; question of legitimacy of work on early universe; mental pictures of the beginning of the universe; first introduction to the flatness problem; visualization of cosmological models; what types of questions can be asked in science; quantum effects in the creation of the universe; different views of quantum mechanics; meaning of time in the early universe; anthropic principle; multitude of universes; questions of purposeful versus accidental nature of our universe.
I’ve got your notes here from the time I talked to you in 1983. You mentioned that a lecture by [Robert] Dicke at Cornell was influential [in motivating your discovery of the inflationary universe model]. Anyway, we don’t need to go through all of that history now. I wanted to talk to you about some more general questions. One thing I wanted to get your opinion about is why you feel the inflationary universe model has caught on so well.
Yes. It’s a good question and I think it has caught on better than I would have expected. To some extent it was just an overdue idea. It really is a rather short step beyond what had been done previously in particle physics, and it answered a lot of questions that people had already realized. It also, I guess, is really the only theory that’s around right now that makes real predictions about what the early universe should look like. The other ideas that people have — most ideas that people have make no predictions.
Don’t you think people could have used the standard [big bang] model without the inflationary epoch to make predictions? I guess the initial conditions would have played a larger role.
The initial conditions would have played a much larger role. And for anything that goes beyond the basic issues, like the density fluctuations, the standard model really makes no predictions. You just have to put those in by hand. So people could have just been resistant to the idea. Actually I am a little surprised that inflation has caught on so well. But if you want to think about the broader questions inflation answers, right now it seems to be the only theory around that answers those questions. It’s also a fact that I had some lucky breaks as far as public relations, as far as the speed at which [the theory] traveled. When I first came up with the idea I was at SLAC [Stanford Linear Accelerator Center]. That year, Sidney Coleman was at SLAC and Lenny Susskind was at Stanford, spending a good deal of time at SLAC. When I gave the seminar, when I first talked about inflation, Coleman and Susskind were in the audience. Both of them got very excited about it and felt right away that it was a good idea. At least initially, as far as the spread of information, I. think they were both instrumental. Both of them went around and talked about it a lot. I was just a lowly postdoc. If I had gone around talking about [the inflationary universe model] no one would have listened for quite some time.
I remember you’re saying you were well aware of Coleman’s work on the false vacuum.
Was that influential in your thinking?
[Long pause] It was an essential piece of background. It doesn’t tell you that much new about the problem [of cosmology].
But he hadn’t discussed the fact that the false vacuum would then give you an equivalent energy density which would then give exponential expansion?
That’s right. He had not done that.
Which is a very critical step. Had he been talking about the false vacuum in the context of cosmology?
He made one cosmological statement in his paper, I guess, about the decay of the false vacuum. The paper was mainly a particle physics paper — the questions of figuring out if you have a false vacuum, how does it decay, which is a very interesting particle physics question. In other words, it was pretty mysterious to people before Coleman’s paper. It was a Russian paper that [first] had it approximately right.
Was that [Andrei] Linde?
No. Coleman cites them. We can look it up. It’s not Linde. Linde had papers about phase transitions in the early universe, but he didn’t talk at all about the mechanism of the phase transition, which certainly would be different in different cases. In Coleman’s paper, after doing this calculation with the WKB formula, he ends by saying — he always words things in very cute ways — he says that numerically the answer is sensitive to a lot of parameters. If it [the age of the universe when the phase transition occurs] turns out to be 10100 years, then you’d never see it and it’s possible that our own vacuum is still unstable. If the lifetime turns out to be a fraction of a second, then there would be a secondary bang [after the primary big bang]. If the lifetime turns out to be 1010 years, then we’d have cause for anxiety.
Yes, I see. So that’s really pretty far from taking it [the phase transition] seriously as an instigator of a new phase of the early universe. One thing that I’m interested in is the way that you visualize the geometry of space and the way that you visualize the beginning of the universe, if you do think about the beginning. But before getting to what your current views are on that, do you have any memories whatsoever of how you thought about cosmology before you became a scientist?
As a child?
As a child, or anytime growing up until you became a scientist. You started out as a particle physicist.
I was an amateur cosmologist.
An amateur cosmologist for a long time. But do you remember any of your earlier views about the universe?
I guess when I was in high school I read at least one book on cosmology. I don’t remember what it was. I remember learning about the possibilities of open and closed universes. Also, whatever book I read talked about an oscillating universe as one possibility, and I think that’s the one I liked the best. It seemed natural.
That [the oscillating universe model] was in fashion for a while there. It doesn’t seem to be too much in fashion right now.
Yes, that’s right.
And you liked the oscillating universe the best?
Yes, from what I remember.
Do you remember why you liked it the best, why it would appeal more?
I guess it had the appeal that you seemingly didn’t have to explain the beginning. And if you can’t explain the beginning, it’s nice not to have to explain the beginning [laughs].
Yes. Because you could imagine that the oscillations occurred for an infinite period of time.
Yes, that is certainly [the way I thought about it.] I didn’t know about the increase of entropy at that time, and all the problems [that brings.]
But that’s only a recent thing that people have been talking about.
So one attraction about an oscillating universe was that you didn’t have to think about the beginning. Did you think about the beginning?
[long pause] I was aware that if you didn’t have an oscillating universe, you’d either need a beginning or a steady state cosmology. I don’t remember what I learned at that time about the steady state cosmology, but I think it was already somewhat on the way out.
You said you read about some of these ideas in cosmology. Was that in high school?
I would guess high school.
That’s amazing. Was it a standard physics textbook, or was it Scientific American, or what?
No, I’m sure it wasn’t a textbook. It was more at the popular level.
So you were reading some of the popular scientific literature?
It’s very possible that it could have been an article in Scientific American. I remember reading articles in Scientific American when I was in high school.
Yes. It could be. I certainly did read Scientific American when I was in high school. In fact, much more thoroughly then than now [laughs].
Yes, I understood much more of it [Scientific American] then than I do now. I guess you could find out what popular magazines were around at that time, whenever it was — the middle 1960s, the early 1960s — and see what it could have been. At that time, did you ever try to imagine how far space extends, or whether the universe was finite or infinite, or any of those kinds of questions?
Yes. That also was a question that came up in some of my popular reading. I did learn about closed universes. I think I had a reasonable, vague understanding of what a closed universe was — I knew about the usual analogy of the two-dimensional surface of a balloon, increasing it by one dimension. It’s hard to know now how much sense I was really able to make out of that idea. But I bought it. I thought it made sense. I doubt if I could have calculated the volume of a closed universe. Probably not.
It’s a little tricky
It’s a little tricky.
I remember there’s a factor of two there that’s a little tricky.
Yes, if you use the Robertson-Walker coordinates there’s a factor of two. If you use the coordinates appropriate to a sphere that factor of two is not a problem.
Do you have any sense of how your cosmological views evolved [after] this high school memory, as you became a practicing cosmologist? Do you recollect any big changes that your views went through? I guess you jettisoned the oscillating universe idea at some point.
Yes. I really did have a rather long vacation from cosmology. It was from the time I was in college or in graduate school. Up until 1979 or so I really did very little thinking about cosmology. Particle physics certainly is a handful in itself. And there was nothing I was doing in particle physics that brought me towards cosmology.
So when you did start thinking about it [cosmology] again in 1979 do you remember how you thought about, let’s say, the beginning of the universe?
Well, OK. One thing I certainly felt was that the universe was a frighteningly open problem, and it was very different from what I had worked on previously in particle physics. Particle physics is a much cleaner subject than astrophysics and cosmology to begin with. And even within particle physics, I had always sought out the cleanest, most mathematically well-formed problems to work on. So there really was a big change for me to start working on something as messy as cosmology. As I may have told you last time, I really got my arm twisted into it by a friend of mine at Cornell, Henry Tye. I certainly wasn’t involved [previously]. Another influence on me at the time was Steve Weinberg, who was a person I respected very much from particle physics.
He was already working on cosmology.
He was already working on cosmology. He was working on exactly this sort of thing, actually — grand-unified theories and cosmology. So if he could work on it, it must make sense.
I see. You felt he gave certain legitimacy to the subject.
That’s right. And for me that was a problem.
Let me ask you about that. When you say it [cosmology] didn’t seem legitimate, do you mean that in practice it was too messy to pin down, but in principle it might have had answers? Or even in principle it was hard to pin down? What do you mean by legitimacy?
Yes, it’s a fair question. I doubt that I would have ever said it’s impossible in principle to pin down cosmology. I think it’s a question of what I thought we were ready to understand at the time.
Did it bother you any to be making theories about the beginning [of the universe]?
You mean in a kind of a religious way?
Just in any kind of way.
It bothered me in the sense that it was so remote; you didn’t know how to test it. To some extent that’s still true. Even if inflation is right, it’s certainly hard to prove its right. But once I started working in it, I did gradually come to the [view] that, even though there are a lot of things we don’t know, it is still possible to make reasonable calculations. What impressed me when I started working on cosmology was the way in which order of magnitude estimates work in cosmology — the fact that if something is wrong, it’s typically not wrong by a factor of 2 but by a factor of 1020. So there were things you could learn — from it, and it made sense to study cosmology at this time, even though it’s not nearly as precise obviously as other physics.
So you felt this [property of huge numbers in cosmology] would allow you to weed out wrong theories more quickly and, if not to find the right theory, at least to know what the wrong theories were.
Did you begin taking more seriously a theory for the beginning of the universe when you started working in cosmology?
When I started working, I accepted the standard big bang model quite seriously, which is what everybody else was doing at the time.
Everybody who thinks about that model has their own picture of the beginning. Do you remember any kind of picture that you had or have now?
The big bang theory of the universe?
[Picture] of the beginning. What it really means.
The view that I might have had, although it’s hard to tell over what time period this view emerged, was that it was practically impossible to take the standard big bang model of the universe seriously back to time zero. Certainly, one reason is that you just know that when the temperature gets to be of the order of the Planck [quantum mechanical] scale, there’s got to be physics going on that you know nothing about. Also, the fact that the universe is so unstable at very early times. The actual mass density must have been incredibly close to the critical density, or else the behavior of the universe would have been very different from the way it was. I’m not sure when I realized it, but [Robert] Dicke and [James] Peebles, in the same article they talk about that [the very close balance between actual mass density and critical mass density, or the “flatness problem”], also talk about the fact that it’s more than just one number that has to be fixed. It’s really a local density and a local expansion rate of the universe; it’s not just the mean values. Every Fourier component basically has to be started out with incredibly small amplitude. In other words, you can look at the density fluctuations in the early universe; they grow linearly with t, so that every Fourier component has to be fine-tuned to incredibly small amplitude at the start, very early, near time zero.
So that made you think that it was very hard to understand what was going on then?
Right. I think initially I believed that the Planck mass must be the cut off, or close to Planck. So initially I think I was going under the assumption that the standard cosmology could go back to the Planck time, and before that it was just too far beyond our present understanding of physics.
Did you have any picture of what it was like at that time?
I rather quickly became indoctrinated with the picture that the universe was smooth and in thermal equilibrium, homogeneous.
When you sit down to solve some equations for the expansion radius — which physically says something — do you have a visual picture in your head? Sometimes I think of a little ball that’s expanding or something. Do you have any kind of picture like that in your head? Maybe you don’t think that way.
I guess sometimes I do. I agree that the ball is about the only picture you can use for the early universe even though it’s not all that accurate. It doesn’t have the same boundaries as the universe. I certainly was not terribly aware of the actual numerical values in cosmology when I started to work on it. I remember a few nights before I gave my first talk [at SLAC] I had to review some material so I would have some idea about what I was talking about. I remember reading Steve Weinberg’s popular book on cosmology.
How do you picture the creation of the universe? Now we think there were quantum mechanical effects. We could have said that a long time ago just on the basis [of] dimensional analysis. Do you have any ideas of how the universe came into being?
When I started working on cosmology, and before I [discovered] inflation, I think I would not have asked that question.
You wouldn’t have asked the question?
Right. You only ask questions that you can get some idea of how to answer.
As a scientist that’s true, but what about just as a person? What were you thinking about, not necessarily [for] posing to other scientists?
I was not thinking about how the universe was created. I do to some extent try to focus my thoughts on questions that I have some tools for approaching. It’s not just that I’m a scientist. How long can you spend thinking about something for which you have no idea what to think? At that time I was not at all aware that it was possible to make a universe from nothing. I would have thought at that time that conservation of energy would make that impossible.
[But] if the universe has zero energy, then conservation of energy doesn’t prevent you from [creating the universe from nothing].
That’s right. But I was not aware at that time that it was a possibility.
How do you think about it now?
Now, I do think that it’s a question that science is beginning to ask. And I think it’s very plausible that the universe is a quantum fluctuation starting from absolutely nothing. Those [calculations] still involve questions that need answering. But all that is very plausible. I have not been following in any great detail the work on the quantum creation of the universe by [Alex] Vilenkin and [Stephen] Hawking et al. I understand there is a disagreement of signs [between the calculations of Vilenkin and Hawking] which I have never tried to understand. I guess I feel that even though we’re beginning to answer those questions, my guess is that without a real theory of quantum gravity, the sign disagreement between Vilenkin and Hawking may be one of those things that is not resolvable unless you have a real theory. My emotional feeling is that we don’t quite yet have the scientific machinery to pin these questions down. But I think we’re certainly working on it and I also think that qualitatively the idea of the creation of the universe out of quantum fluctuations is so simple that it has a very good chance of being right.
Let me ask a question about that. If the universe began as a quantum fluctuation, what was there before the fluctuation? How do you think about that?
OK, I can tell you the image I use for that, although I’m not sure it will hold up in time. What I think now is that in the hypothetical theory of quantum gravity, which doesn’t quite exist, there will be a Hubert space of all the possible states of the universe and the different states of the universe will clearly have different possible geometries for the universe, and among all of those states and those possible geometries I assume that one of the possibilities is a universe with zero points — [in other words], a closed universe with its radius equal to zero or something like that. That’s what I sort of take as the definition of “nothing,” and that’s what I take as the starting point, a universe with no points. I’m assuming that the theory of quantum gravity will supply a mechanism for calculating the transition amplitude [probability] from that state to other states.
How do you visualize all of these different possible universes before this particular universe [came into being]? Would these amplitudes be changing in time? I know the whole idea of time is difficult to talk about.
Right. I’ll buy your last statement that we can’t really talk about time in this quantum universe. What I would guess is correct is that the only thing that exists in the theory is this: given a description of a space like slice, there’s a recipe for calculating the amplitude for that space like slice to exist. The notion of time in this description arises in a very peculiar way. The wave function doesn’t depend on time; it’s just a function of the slices. The idea [is] that even if you had a whole history of the universe, it would just look like so many [space like] slices lying out on the table, rather than [having] a particular order [in time].
Each one [spacelike slice] would have some amplitude.
Each one would have some amplitude. Right. And the sense of continuity [time] that we perceive in nature is really just a correlation… that is, if you have a slice of the whole universe there will be devices in that slice that you can think of as clocks, including clocks themselves, and the “flow of time” is just a strong correlation between what a clock is reading and something else that is going on.
So you think that even in the current universe, which is in some sense in the classical [non-quantum mechanical] domain, the concept of time is artificial. Maybe artificial is not the word.
Well, a classical approximation. [The concept of time] is certainly a good approximation. But as I understand it, in a full quantum gravity description of the universe, the concept of time would arise simply as a correlation...
Between different slices...
In the amplitudes.
So if you had two slices that were very similar [in spatial structure], then you would associate them as being close together in time.
That’s how you would define time, that’s how you would experience it.
In this quantum era, how do you picture these different possible universes?
The view of quantum mechanics that has always made the most sense to me — although I. admit that there are some features that don’t make sense to me — is the Everett-Wheeler view, the “many-worlds interpretation,” where you think of all the components of the wave function as representing reality, and our reality is just a branch that we happen to be on. But it’s hard to know what words to use. One could at least imagine that there are people on other branches of the wave function — who could be sitting [somewhere] having a conversation just like the one we’re having now.
So all of these possible universes in some way exist.
Then what meaning do you give to the words “the universe came into being?” Does that just mean that a particular universe changed from having zero points, from having zero radius, to having a finite radius?
Actually it probably has no precise meaning. I know Hawking tries to avoid talking about [the beginning] for that reason. I think it has meaning to the extent that this classical interpretation of quantum mechanics gives a sense that time flows. But my understanding of quantum gravity, which is certainly not perfect, seems to indicate that there’s a level of quantum gravity [in which time cannot] be described. If you follow the universe backwards in time toward the origin of the universe, you don’t need quantum gravity while time is evolving, but I think that once you get down toward the beginning of the universe, where you really need quantum gravity, my guess is that Hawking is right, that you really should not talk about the universe beginning at a time. The boundary conditions determine the wave function of the universe.
But in that picture I take it that you would also imagine that there is other universes simultaneously existing.
Yes, that’s right. In my view, there are other universes that are simultaneously existing. And there are two levels of other universes that are simultaneously existing. If inflation is right, even in our component of the wave function, the universe is so huge that there are parts that are totally disconnected from us.
Yes, I understand that.
But in addition, there would be other components of the wave function superimposed on our part. And in the many-worlds interpretation these other components would also be interpreted as real universes, although it’s hard to define in what sense they would be real. I think if really pressed to defend what it is that I’m saying, then I’d step back and say as little as possible, and just say what the quantum theory would predict and that’s all there is to it. But then I would add that if one wants to have a picture of quantum mechanical [reality] to make sense out of, the picture that makes the most sense to me is that these other universes are also real, and the people living there are as real as you and me.
What do you think about the anthropic principle? I know there are many versions of it, from something relatively mild to Wheeler’s Observership idea, which is the most extreme form. Where do you put yourself in that discussion?
Emotionally, [the anthropic principle] kind of rubs me the wrong way. I’m even resistant to listening to it. Obviously, there are some anthropic statements you can make that are true. If we weren’t here then we wouldn’t be here. Some of what people talk about follows from that. As far as the anthropic principle as a way of approaching things, I find it hard to believe that anybody would ever use the anthropic principle if he had a better explanation for something. I’ve yet, for example, to hear an anthropic principle of world history. Historians don’t talk in those terms. They have more concrete things to say. I tend to feel that the [physical constants] are determined by physical laws that we can’t understand [now], and once we understand those physical laws we can make predictions which are a lot more precise.
So you would argue that intelligence, or life, doesn’t have any special role in the universe.
That’s right. I don’t think that intelligence or life has any special role. The fact that intelligence or life has evolved may tell us something [about the universe]. But I don’t think the laws were contrived in order to allow life to exist. I also think that we don’t understand life that well. So it is a rather poor way to try to determine the laws using the fact that life exists. The anthropic principle is something that people do if they can’t think of anything better to do.
One of the ideas of the anthropic principle, which doesn’t necessarily have to be associated with life, is whether the universe is accidental or not. In your view of the quantum mechanical origins of the universe, [would you] say that these other universes might have different dimensions of space or different values of the fundamental constants?
Yes. Certainly in my grand wave function, there are many different things going on, so any one branch might be accidental, although [they all have] the same physical laws. So this same picture of the universe could be considered accidental.
When you look at the whole wave function, you would say that’s not accidental?
That’s right. The whole wave function is not.
So our particular branch is just one that we happen to be on; there are other people, other creatures possibly living on other branches.
Right. And one of the questions people worry about is what it is that determines the laws of physics. Of course, there are a few coincidences. If the neutron lifetime weren’t what it is, the universe would be either all helium or else it would contain almost no helium. Things like that. My emotional feeling is that the general [laws] are really very restrictive, including how the neutron lifetime comes out and my guess is that there really is only one consistent theory of nature that has no free parameters at all.
Why are the laws of physics so restrictive?
That I don’t claim to know. The things I mean [by the laws of physics] are the general notions of quantum theory and relativity. It is very difficult to combine these two sets of ideas, and there may be only one consistent way of doing it.
Do you think there’s a possibility that some intelligence created the laws of physics, and then let things evolve from there?
It is certainly a hard question you ask. We really don’t know where this wave function sits. It could be on somebody’s Cray [computer] [laughs]. I’m not sure whether creatures living inside some numerical simulation on a Cray are [real], although some people tend to think in those terms.
One of the overtones of the anthropic principle is that the universe has some kind of a purpose.
And if you thought that the laws of physics were so restrictive that they could not have been accidental, then that would suggest a purpose to the universe.
I don’t know. I would say that another way if there were ziffions of possible universes, and our universe happened to have a neutron lifetime in the range to allow life to form — that I think would suggest a purpose. The alternative point of view is that there is only one consistent way the universe could have evolved.
Maybe I misunderstood what you said earlier, but I understood you to say that in this sort of pre-existence, this Hilbert space which was the soil from which emerged our universe, that there were other universes with finite amplitudes [probabilities] which had very different properties from our universe. Now when you think of those different properties, are you talking about little details, or are you talking about the number of dimensions of space, or the speed of light?
I’m not sure I know. My instinct is that the laws of physics at the most fundamental level are universal and apply to all branches of the wave function. Yet it’s hard to determine what that level is from the apparent level. Years ago people tended to see the laws of physics just being as God made them. But in the current climate, it’s much easier to believe that what we see is to some extent just a particular state that our universe happens to be in. So that the details might be different [in other possible states or branches].
Would [such details] include the number of dimensions of space, for example?
Very likely. In some sense, the absolute number of dimensions could be fixed [the same in all branches of the wave function]; it may be 26 or something like that. But the apparent number could be much smaller due to compactification. It [the number of dimensions of space] could then be different in different universes. On the other hand, it could be that only four dimensions are stable. I’m not saying that I believe that other dimensions are possible; I’m just saying that it is conceivable to me.
On the question of the lifetime of the neutron — which we agree might have some relationship to the ability of life to emerge, although we don’t know how restrictive that is — do you think that other universes that have the same laws of physics but might have different details, [such as the] lifetime of the neutron? Of course it [the neutron lifetime] would have to be relative to something else. [The significance of] an absolute number like that must be [considered] relative to something else.
Yes, it would have to be relative to something else; somebody’s watch [laughs]. Usually when you have spontaneous symmetry breaking, you have very different types of vacuums of the universe. So I would think it unlikely that there would be other universes that would be just like ours, except that the [fundamental] constants would be slightly different.
So to that extent, our universe would be just accidental.
Because a minute ago you were turning around the anthropic argument. I can’t remember exactly what you said, but you said that the anthropic argument would have some significance if we could imagine [a situation in which there were] lots of different universes, but only this one would permit life. But if the laws of physics greatly restricted the kind of universe to be a universe very much like ours, then this is the only universe we could have so it doesn’t make any sense to talk about how improbable this one is. How do you reconcile that [argument] with what you just said? I’m not trying to pin you down; I just want to understand what you’re saying.
Right. One of the things I’ve been thinking is that if the laws of physics do appear to be different in other places [or universes] with different [forms] of spontaneous symmetry breaking, then they will be very different. It’s not just a question of a small difference. That’s important in terms of the anthropic principle. Because, for example, if the value of alpha [the fine structure constant] were continuously variable, then the anthropic people would come along and say that it has the value it has because otherwise life wouldn’t have formed. So there may be only one universe that would allow life to form. I just don’t think that alpha has continuous values. There may be universes where it has the value 1/137 [like in our universe] and other universes where it has the value 2. It’s still true; I guess that the anthropic principle might make some sense in terms of selecting certain types of spontaneous symmetry breaking. In some cases, spontaneous symmetry breaking may lead to physics that does not lead to life. I don’t know. I don’t think much about that. That is a question for the future. At the moment, I don’t think we are at all prepared to even say what the laws of physics would be in these other branches [parts of the overall wave function]. When we do get to the point where we can talk about that, I think it will have to be in terms of some fundamental theory like the superstring theory. And again, it probably won’t be the anthropic principle that tells us what the vacuua look like.
Why do you call [states of the universe] vacuua?
Good question. I call them vacuua because in the theory of spontaneous symmetry breaking we believe that even if there are phase transitions, the phase that we’re in is a phase of zero energy or a minimum energy state.
And you call that the vacuum.
I call that the vacuum. It may be a false vacuum.
So that vacuum is a minimum energy state.
Yes, at least a local minimum.
Maybe that’s a good place to stop.
 November, 1978
 S. Coleman “Fate of the False Vacuum: Semiclassical Theory,” Physical Review D, vol. 15, pg. 2929 (1977)
 M.B. Voloshin, I. Yu Kobzarev, and L.B. Okun’, vol. 20, pg. 1229 (1974); trans in Soviet Journal of Nuclear Physics, vol. 20, pg. 644 (1975)
 R. H. Dicke and P.J.E. Peebles, “Enigmas and Nostrums in Cosmology,” in General Relativity: An Einstein Centenary Survey, ed. S.W. Hawking and W. Israel (London: Cambridge University Press, 1979)
 The First Three Minutes (New York: Basic Books, 1977)
 A.H. Guth, “Inflationary Universe: A Possible Solution to the Horizon and Flatness Problems," Physical Review D, vol. 23, pg. 347 (1981)
 H. Everett III, Reviews of Modern Physics, vol. 29, pg. 454 (1957); J.A. Wheeler, Review of Modern Physics, vol. 29, pg. 463 (1957)
 C.W. Misner, K.S. Thorne, and J.A. Wheeler Gravitation (San Francisco: Freeman, 1973), chapt. 44; J.A. Wheeler “Genesis and Observership,” in Foundational Problems in The Physical Sciences, eds. Butts and Hontikka (Dordrecht: Boston, 1977)